Conventional binoculars are focused on objects at different distances from the viewer by rotating a round knob or focus wheel which adjusts the position of ocular lenses until the magnified image appears sharp. Many binocular users have difficulty in making this adjustment because the focus knob is slow and cumbersome to operate, and often requires the hands to be awkwardly shifted from a normal gripping position in order to rotate the knob.
Precise focus is also difficult for many people to achieve in a single adjustment due to the inherent tendency of the eye to "force" a slightly de-focused image into focus. This action arises from an unconscious distortion of the eye's crystalline lens to achieve sharpness of the retinal image. The crystalline lens will "lead" a slow focus adjustment, and accurate adjustment is sometimes not accomplished until the binocular has been removed from and returned to the eyes several times.
These problems are recognized and discussed in greater detail in U.S. Pat. No. 3,540,792, the disclosure of which is incorporated herein by reference. The solution shown in this patent is a short-stroke focus ring which is rotatably mounted on one of the ocular-lens housings.
I have found that many binocular users are better able to achieve accurate focusing if the fingers of both hands are used to make the adjustment, and if the fingers apply opposing forces to the control or focus knob of the focus mechanism. The fingers of the user's one hand apply a force to drive the control knob in one direction, and the fingers of the user's other or second hand apply an opposite retarding force which tends to minimize overshoot and makes the adjustment smooth.
If the control knob is driven past a position of sharp focus, the fingers of the second hand are positioned to return the knob to a correct position immediately and without any shift in grip positioning of the hands. Even persons skilled in the adjustment of optical instruments find this opposed-force arrangement comfortable because they typically tend to make several rapid sweeps of decreasing amplitude through sharp focus in order to zero in on a correct adjustment.
The binocular herein described uses a centrally positioned focus control knob which differs from a conventional center-focus wheel in several ways. First, the knob is coupled to the ocular lenses by a steep cam or similar means which provides ocular movement through a full focusing range with only a relatively small movement of the knob. Second, the upper surface of the control knob is generally flattened and laterally extended to form a treadle- or seesaw-like "rocker" knob having platform surfaces which fall naturally under the user's fingertips when the binocular is held in the normal way. These surfaces are on opposite sides of the axis of rotation of the knob so movement of the fingertips is opposite and dominantly vertical during focus adjustment, and accurate focus is achieved without loss of the steadying grip of both hands on the binocular. In another form, the finger-contacting platform surfaces are arranged as substantially parallel opposite side surfaces of a tab which extends radially from the knob rotation axis.